FYI This is also a problem with Access 2010 the solutions still work.
I caused the problem by moving the divider between the navigation panel and the form/design/ panel to the left as far as it would go. I lost the "wide" line that I could use to make it wider again.
I was in a hurry (deadlines!) so tried exporting everything to a new database to continue on working. That was OK for then.
Today I had more time and went looking again for the real fix. And F11 is it.